In today's chips, memory consumes a significant amount of chip area and is always a bottle neck which limits chips to run at lower operation voltage and higher speed. For example, SRAM (static random access memory) always occupies large operation resources and leads to serious metal layer routing congestion problem in P&R (placing and routing) design.
The physical implementation of SRAM in industry generally uses a single-driven mode, i.e. address decoding and driver logic are arranged on one side of a whole memory array, and word line is decoded to output throughout the entire memory array. After the process below 65 nm, RC delay of word line itself has seriously influences the performance and the yield of SRAM. Due to a proportional relationship between RC delay and routing length, circuit design needs that word line of a long distance routing is cut into shorter word line so as to improve the slope and delay of the word line.
In prior art, a center-driven mode is always considered to be used to decrease the word line length, for example, a whole memory array can be divided into two smaller memory arrays, between which routing can be arranged. However, central routing of the center-driven mode is very intensive, the routing design is complex, and common control logic and chip area cannot be used sufficiently. Another implementation of prior art, as shown in FIG. 1, uses a global-driven mode that is based on the center-driven mode. In SRAM 100, word line decoding and routing is performed on a upper metal layer (for example, M5), including pre-decoding by pre-decoder 110 and final-decoding by final decoder 120, and then the output word line, on a metal layer (for example, M3) below M5, drives memory array 141 and memory array 142. Although the global mode solves some problems above mentioned, the upper metal layer becomes very intensive, and because signals of power and ground is also on the upper metal layer, word line needs to be shielded.
In the above-mentioned process, inventor found that the prior art has at least the following problems:
To improve the quality of word line signals results in routing congestion.